JP2005272993A - Method for electrolyzing alkali metal chloride - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for electrolyzing an alkali metal chloride including a saturated brine preparation stage, an electrolysis stage, a chlorine degassing stage and a chlorine adsorption/removal stage with active carbon, improved in such a manner that the prevention in the reproduction of halobacteria in the chlorine adsorption/removal stage can be attained. <P>SOLUTION: A water feeding means is provided in a chlorine adsorption/removal stage to reduce the concentration of a salt in water contacted with active carbon. In a preferable embodiment after the chlorine adsorption/removal stage, as a Glauber's salt removal stage, the stage wherein plain brine and water are alternately passed into a separation column filled with an ion exchanger, an effluent mainly comprising an alkali metal chloride and an effluent comprising a sulfate are separated, so as to be recovered, further, the former is circulated through the saturated brine preparation stage and the latter is removed to the outside of the circulation system is provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an alkali metal chloride electrolysis method, and more particularly, an alkali metal chloride electrolysis method including a saturated brine preparation step, an electrolysis step, a chlorine deaeration step, and a chlorine adsorption removal step by activated carbon, The present invention relates to an industrially advantageous electrolysis method improved so as to prevent the growth of halophilic bacteria in the adsorption removal step.

  Conventionally, alkali metal chloride electrolysis methods including a saturated brine preparation step, an electrolysis step, a chlorine deaeration step, and a chlorine adsorption removal step by activated carbon are known.

For example, a saturated brine preparation process in which alkali metal chloride containing sulfate as an impurity is dissolved in water, an ion exchange membrane type electrolysis process in which saturated brine is electrolyzed, the concentration of alkali metal chloride extracted from the electrolysis process Chlorine degassing step for deaerating chlorine from the reduced brine, the effluent containing mainly alkali metal chloride, with the light brine and water alternately passed through the separation tower packed with the ion exchanger. And an effluent containing sulfate, and the former is circulated to the saturated brine preparation step and the latter is removed from the circulation system, and the alkali metal chloride electrolysis method includes a denitrification step to be removed from the circulation system. Provided with a chlorine adsorption removal step for adsorbing and removing residual chlorine in the light brine derived from the chlorine deaeration step with activated carbon between the chlorine deaeration step and the degassing step Genus electrolytic process of chloride are known.
JP2001-181877

  In order to solve the problem that the ion exchanger in the degassing step deteriorates due to chlorine in the light brine remaining without being removed in the chlorine degassing step, the above electrolysis method is used in the chlorine degassing step and the degassing step. This is an improved method in which a chlorine adsorption removal step is provided between them.

  The chlorine adsorption removal process in the prior art is significant not only from the viewpoint of preventing the deterioration of the ion exchanger in the degassing process but also from the viewpoint of preventing pipe corrosion, but the following problems have been found. That is, halophilic bacteria suitable for light brine grow in the activated carbon packed bed of the chlorine adsorption tower, such bacteria and their waste products flow out of the chlorine adsorption tower, block the piping and equipment in the subsequent stage, and the operating pressure Increase the performance and decrease the performance of the stripping process.

  The present invention has been made in view of the above circumstances, and its purpose is to provide an alkali metal chloride electrolysis method including a saturated brine preparation step, an electrolysis step, a chlorine deaeration step, and a chlorine adsorption removal step using activated carbon. It is an object of the present invention to provide an alkali metal chloride electrolysis method improved so as to prevent the growth of halophilic bacteria in the adsorption removal step.

  As a result of intensive studies, the present inventors have found that halophilic bacteria adapted to light brine cannot be grown in a low concentration salt environment and die.

  The present invention has been completed based on the above findings and has been completed. The gist of the present invention is a saturated brine preparation step for dissolving an alkali metal chloride containing sulfate as an impurity in water, and electrolyzing the saturated brine. In an ion exchange membrane type electrolysis process, a chlorine deaeration process for degassing chlorine from a light brine extracted from the electrolysis process and having a reduced concentration of alkali metal chloride, and a light brine derived from a chlorine deaeration process In an alkali metal chloride electrolysis method including a chlorine adsorption removal process for adsorbing and removing residual chlorine with activated carbon, the salt concentration of water in contact with activated carbon in the chlorine adsorption removal process by providing a water supply means in the chlorine adsorption removal process The present invention resides in a method for electrolyzing an alkali metal chloride, characterized in that it lowers.

  In a preferred embodiment of the present invention, after the chlorine adsorption removal step, as a degassing step, light brine and water are alternately passed through a separation tower packed with an ion exchanger, and mainly alkali metal chloride is added. The effluent contained and the effluent containing sulfate are separated and recovered, and the former is circulated to the saturated brine preparation step, and the latter is removed from the circulation system.

  According to the present invention, the growth of halophilic bacteria in the chlorine adsorption removal process is prevented, so that bacteria and waste products do not flow out from the chlorine adsorption tower, the piping and equipment in the subsequent stage are blocked, and the operating pressure is reduced. It is possible to solve the problem of the increase and the performance degradation of the dehulling process all at once.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a process explanatory view showing an example of the electrolysis method of the present invention. In the figure, reference numeral (1) is a dissolution tank for a saturated brine preparation process, (6) is a purification facility provided as necessary, (7) is an electrolytic tank for an electrolysis process, and (13) is a chlorine degassing process for a chlorine deaeration process. A gas tower, (15) is a separation tower in the degassing step provided as a preferred embodiment, and (23) is a chlorine adsorption tower in the chlorine adsorption removal step.

  The electrolysis method shown in FIG. 1 is provided with a degassing step after the chlorine adsorption removal step, and from the viewpoint of reducing the amount of reducing agent used in the chlorine adsorption removal step, the light brine recovered from the chlorine deaeration step Is partly supplied to the degassing step, and is basically the same as the electrolysis method described in JP-A-2001-181877.

  First, a raw salt of alkali metal chloride (containing sulfate as an impurity) sent from the feed path (2) in the dissolution tank (1), dissolved water from the branch pipe (3a) of the conduit (3), A high-concentration salt water to be supplied to the electrolytic cell (7) is prepared using a fraction solution from which the light brine and sulfate circulated from the electrolytic cell (7) are substantially removed. The above fraction liquid refers to an alkali metal chloride-containing fraction liquid from which sulfate has been substantially removed by the separation tower (15). That is, after the electrolytic treatment of the alkali metal chloride aqueous solution in the electrolytic cell (7), the alkali metal chloride aqueous solution (light brine) in which the content ratio of the alkali metal chloride discharged from the electrolytic cell (7) is lowered is reduced. A liquid fractionated according to a chromatographic method and treated so as to contain substantially no sulfate and contain an alkali metal chloride, and this fraction liquid is passed through a conduit (4) to a dissolution tank (1). Derived.

  The alkali metal chloride concentration in the salt water is preferably as high as possible, and normally saturated salt water (saturated brine) is prepared. The saturated brine prepared in the dissolution tank (1) is sent to the purification facility (6) through the conduit (5) to remove impurities such as calcium salt, magnesium salt and strontium salt in the saturated brine derived from the raw salt. The As a purification method, for example, a method in which sodium carbonate and caustic soda are sequentially added and precipitated as calcium carbonate and magnesium hydroxide is employed. If necessary, a purification method such as a chelate resin treatment may be used in combination. Next, the saturated brine is sent to an ion exchange membrane type electrolytic cell (7), and electrolysis is performed according to a conventional method.

  The electrolytic cell (7) is divided into a cathode chamber and an anode chamber by a diaphragm (20) made of an ion exchange membrane. Caustic alkali generated in the cathode chamber is from the conduit (11), and hydrogen gas is from the conduit (9). Chlorine gas discharged and generated in the anode chamber is discharged from the conduit (8) and recovered. In addition, the code | symbol (10) is a conduit | pipe of the water supplied in order to extrude caustic from an electrolytic cell (7). The electrolysis consumes about 50% of the alkali metal chloride and about 20% of the water in the saturated brine, and the remaining brine (fresh brine) is withdrawn from the conduit (12) and circulated to the dissolution tank (1). The light brine extracted from the electrolytic cell (7) usually contains about 180 to 200 g / L of alkali metal chloride and about 6 to 12 g / L of sulfate. The pH is usually about 2 to 4. Such light brine is preferably deaerated by aeration in a chlorine deaeration tower (13) after pH adjustment to 1-2 with hydrochloric acid, for example. Then, if necessary, after being stored in a storage tank (not shown), at least a part thereof is sent to the chlorine adsorption tower (23) via the conduit (24) by opening a valve (14a). Then, it is fed to the separation tower (15) through the conduit (25).

  In said chlorine adsorption tower (23), granular activated carbon is used suitably, and the particle size is 0.5-10 mm normally, Preferably it is 1.0-5.0 mm. The activated carbon is packed in the chlorine adsorption tower (23) at a tank height of 0.5 to 4.0 m. And the liquid flow is normally performed by a downflow, the speed is usually 5.0 to 25 m / h in linear velocity, and the liquid flow temperature is usually 50 to 90 ° C.

  The separation column (15) is packed with an amphoteric ion exchanger having an anion exchange group and a cation exchange group, and these both ions forming an internal salt. Examples of the amphoteric ion exchanger forming the internal salt include a resin generally called a snake cage type. The snake cage resin is a composite obtained by impregnating and polymerizing acrylic acid with a strong basic ion exchanger of styrene or acrylic, and “Rita Deion 11A-8” manufactured by Dow Chemical Company, “ It is marketed under names such as “Diaion SR-1”. In addition, as the amphoteric ion exchanger forming an internal salt, a resin having an ion exchange group represented by the following general formula (1) directly bonded to a resin matrix composed of an acrylic or styrene cross-linked copolymer Also used.

(Wherein R ¹ and R ² each represent an alkyl group having 1 to 3 carbon atoms, and m and n each represent a number 1 to 4)

  A resin having an ion exchange group represented by the general formula (1) is prepared by introducing a halomethyl group into a styrene-based cross-linked copolymer according to a method described in, for example, Japanese Patent Publication No. 60-45942. It is produced by a method in which an acid derivative of N-substituted amino acid such as acid anhydride, acid amide, acid halide, lower alkyl ester or the like of N′-dimethylglycine is reacted, followed by hydrolysis and a method analogous thereto.

  The amphoteric ion exchanger has a spherical shape, and the particle size thereof is usually 100 to 1200 μm, preferably 150 to 350 μm. The exchange capacity for forming the internal salt of the amphoteric ion exchanger is usually 1 to 6 meq / g resin, preferably 2.0 to 4.5 meq / g resin. Moreover, the water | moisture content of an amphoteric ion exchanger is 20 to 80 weight% normally, Preferably it is 30 to 60 weight%. The bed height of the amphoteric ion exchanger packed in the separation tower (15) depends on the type of ion exchanger and the amount of treated water (capacity of the electrolysis plant), but is usually about 1 to 4 m.

The amount of the fresh brine supplied to the separation tower (15) is determined in consideration of the amount of sulfate in the raw salt, the concentration of sulfate allowed in the saturated brine supplied to the electrolytic cell (7), and the like. The sulfate in the light brine does not need to be completely removed and may be maintained at a concentration that does not hinder electrolysis. What is necessary is just to prevent the further accumulation | storage of the sulfate to a saturated brine by removing the amount of sulfate accompanying the raw salt newly added at least. For that purpose, the brine circulated to the electrolytic cell (7) may be treated in the separation tower (15). The amount of the fresh brine supplied to the separation tower (15) is 0.1 to 0.5 times the packed ion exchanger volume, and the water flow rate is ¹ to 5 h ^{−1 in} space velocity (SV). Good. The water passing temperature is preferably 40 to 80 ° C. After passing a single amount of fresh brine, the valve (14a) is closed, the valve (14b) is opened, water is passed from the conduit (3b) to the separation tower (15), and the sulfuric acid is first added according to the chromatographic technique. The salt is eluted and flushed out, and then the alkali metal chloride is eluted and flushed out. As this water, it is preferable to use a solution obtained by filtering dissolved water from the conduit (3) with a filter of about 1 micron, or soft water obtained by softening it. The amount of water flow at one time is 0.2 to 1.1 times the packed ion exchanger volume, and the water flow rate is usually ^{1 to 5} h ^-1 in SV, preferably the same as the water flow rate of the above-mentioned light brine. Good. The water flow temperature is 40 to 80 ° C., and the water flow temperature is preferably the same as that of the light brine.

  The flow direction of the fresh brine and the dissolved water may be either an upward flow or a downward flow when the packed ion exchanger is packed so as to completely fill the separation tower (15). Is filled with leaving an empty portion, the water is passed downward so that the packed ion exchanger does not flow. The change in the component concentration in the liquid flowing out from the separation tower (15) is detected by a detector (16) comprising a conductivity meter, a refractometer or the like, and a valve (18) is transmitted by transmission paths (21) and (22). ) And (19), and the effluent fraction mainly containing sulfate is discharged out of the system via the effluent pipe (17), while the effluent fraction mainly containing alkali metal chloride is discharged into the conduit ( It circulates to a dissolution tank (1) through 4). An operation of opening the valve (14a) again to supply water of light brine and then passing water of dissolved water is repeated. In the dissolution tank (1), the fresh brine circulated directly from the conduit (12) without going through the separation tower (15), the alkali metal-containing effluent fraction from the separation tower (15), and if necessary, further dissolved water Add fresh salt and prepare saturated brine. The light brine adsorbed on the amphoteric ion exchanger is easily eluted with water and can be used again for the treatment of the light brine without regenerating with acid or alkali.

  If the treatment amount of the light brine is an appropriate amount, accumulation of new sulfate due to addition of the raw material salt can be prevented, and water used for elution of the amphoteric ion exchanger is consumed in the electrolytic cell (7). The amount of water used for electrolysis can be circulated to the dissolution tank (1) in the entire amount of the effluent fraction consisting mainly of alkali metal chlorides flowing out from the separation tower (15). In addition, the amount of alkali metal chloride leaking out of the system can be minimized. In addition, the amount of waste water is small and economical.

  In the case of the example described above, as a degassing step that does not use a chemical such as an alkali, that is, a dewatering step in which light brine and water are alternately passed through a separation column packed with an ion exchanger, Although the chromatographic separation mode desorption process described in Japanese Patent No. 3485 is adopted, the adsorption / desorption mode desorption process may be used in the present invention. In such an embodiment, an appropriate ion exchange resin that can be regenerated with water (desorption of sulfate radical or sulfate adsorbed on the resin) is used. Since the adsorptive power of such ion exchange resins to sulfate radicals or sulfates is generally weak, the regeneration before reaching the breakthrough point, that is, the cycle of water that is alternately passed through the light brine is the chromatographic separation mode described above. It is often the same as in the case of.

  The greatest feature of the present invention is that, in the alkali metal chloride electrolysis method as described above, a water supply means is provided in the chlorine adsorption removal step to reduce the salt concentration of water in contact with the activated carbon in the chlorine adsorption removal step. is there. That is, in the case of the illustrated example, the branch conduit (26) with a valve (28a) is connected to the conduit (24) of the chlorine adsorption tower (23), and the branch conduit (27) with a valve (28b) is connected to the conduit (25). Further, a valve (28c) is provided at a position downstream from the branch pipe (27) connecting portion of the pipe (25), the valves (14a) and (28c) are closed, and the valves (28a) and (28b) are closed. ) Are opened, water is supplied from the conduit (27) and discharged from the conduit (26).

  By reducing the salt concentration of water in contact with the activated carbon of the chlorine adsorption tower (23) as described above, halophilic bacteria that have been grown in the activated carbon packed bed can be killed. And when washing | cleaning with the water of an upflow as mentioned above, there exists an advantage which can carry out the floating separation of the dead bacteria with the waste product of the bacteria deposited in the packed bed of sex charcoal. Of course, the water washing may be a down flow. Although the supply amount of water is not particularly limited, the volume ratio is 2 to 10 times that of the activated carbon in the chlorine adsorption tower (23) in consideration of the cleaning effect. The linear velocity of water passing through is usually 10 to 25 m / h. The frequency of such washing treatment is appropriately determined in consideration of the growth rate of halophilic bacteria, but is usually once every 1 to 6 months, preferably once every 2 to 3 months. Prior to the above washing treatment, the light brine staying in the chlorine adsorption tower (23) can be pushed out with air. Specifically, the valves (14a) and (28c) are closed, the valves (28a) and (28b) are opened, air is supplied from the conduit (26), and light brine is supplied from the conduit (26). Extrude and collect.

  In the present invention, as in the electrolysis method described in JP-A No. 2001-181877, the light brine derived from the chlorine degassing step is divided into two, and 3 to 50% is introduced into the chlorine adsorption removal step. The residue is preferably recycled directly to the aforementioned saturated brine preparation step.

  That is, as described above, it is possible to prevent the deterioration of the ion exchanger in the desulfurization process by providing the adsorption removal process of residual chlorine in the light brine, but it is necessary to supply the entire amount of the light brine to the desulfurization process. Therefore, the residual chlorine may be adsorbed and removed with respect to the amount of light brine that is commensurate with the amount supplied to the degassing step. The amount of the light brine of 3 to 50% is an amount determined from this viewpoint.

If the amount of light brine introduced into the chlorine adsorption removal process is less than 3%, the amount of light brine to be degassed while preventing the deterioration of the ion exchanger is too small. The amount of Na ₂ SO ₄ ) increases, and as a result, problems such as deterioration of the ion exchange membrane and increase in power consumption are caused in the electrolysis process. On the other hand, if the amount of light brine introduced into the chlorine adsorption removal process exceeds 50%, an unnecessary amount of light brine is processed, and various chemicals used in the chlorine adsorption removal process (reducing agent, pH adjustment) Etc.) and ion exchangers used in the degassing process are unnecessarily large, which is not economical. The preferable amount of light brine introduced into the chlorine adsorption removing step is 5 to 30%, and a more preferable amount is 5 to 15%.

  Furthermore, in the present invention, it is economical to use industrial water as water to be passed (eluent in the case of chromatographic separation or resin regenerated solution in the case of adsorption / desorption) in the above-described degassing step. And preferred. Here, industrial water refers to water obtained by treating sprayed water with a flocculant such as polyaluminum chloride (PAC) or sulfuric acid band in addition to river water and groundwater. Such industrial water generally contains suspended substances mainly composed of algae and colloidal metals together with a hardness component mainly composed of metal salts. The hardness component is mainly composed of divalent ions such as Ca and Mg, and is expressed in terms of calcium carbonate equivalent. The amount of industrial water hardness here is usually 30 to 200 mg / L calcium carbonate equivalent. Moreover, the quantity of a suspended substance (SS component) is represented as SS component capture | acquired by a glass fiber filter, and is a kaolin turbidity normally 1-10 degree | times.

  In the case of the denitrification process using industrial water, it is more preferable to operate as follows. That is, the operation is stopped when the performance of the ion exchanger is lowered, and the operation is restarted after performing the acid cleaning, alkali cleaning, or alternate cleaning of the ion exchanger. Here, the point in time when the performance of the ion exchanger is lowered generally means the point in time when the chromatographic separation performance by the amphoteric ion exchanger is lowered in the case of the chromatographic separation mode, and in the case of the adsorption / desorption mode. It means the time when the exchange resin reaches the breakthrough point. Such a decrease in the performance of the ion exchanger is caused by contamination of the ion exchanger coating due to the hardness component or deposition contamination in the ion exchanger packed layer due to suspended substances.

  The above-mentioned acid cleaning is applied in the case of coating contamination due to hardness. And HCl is used suitably as an acid, The density | concentration is 10-150 g / L normally, Preferably it is 50-100 g / L. The waste water after the washing treatment is discharged out of the system and then washed with water.

  The above alkaline cleaning is applied in the case of sedimentation contamination by suspended substances. And as an alkali, in the case of salt electrolysis, NaOH aqueous solution is used suitably, and the density | concentration is 1-100 g / L normally, Preferably it is 5-50 g / L. The waste water after the washing treatment is discharged out of the system and then washed with water.

  As described above, even if the acid cleaning and / or alkali cleaning in the degassing step is periodically performed, the cleaning interval is long, so that the cost for the medicine is very low, and the industrial advantage Will not be damaged.

  The present invention is carried out as described above. The following is an example of a characteristic portion of the present invention, that is, an example of a chlorine adsorption removal step for adsorbing and removing residual chlorine in light brine with activated carbon. Although it shows with a dehulling process, this invention is not limited to a following example, unless the summary is exceeded.

Example 1:
Pale brine was drained from the electrolytic cell (7) at 45 m ³ / h, the pH was adjusted to about 2, aerated in the chlorine degassing tower (13), 61 mg-Cl / L free chlorine and 5.0 g / L A pale brine containing sulfate radicals was obtained. Then, about 3.6 m ³ / h (about 8% of the whole) of the obtained light brine, the pH was adjusted to about 11 again, and chlorine adsorption filled with activated carbon 5.0 t having an average particle diameter of 5 mm was used. The solution was passed through the column (23) (inner diameter 2600 mm × length 6500 mm) in a down flow. The liquid temperature was 60 ° C. When the free chlorine in the light brine flowing out from the chlorine adsorption tower (23) was measured, it was less than 0.10 mg-Cl / L, and no change in the sulfate group concentration in the light brine was observed.

Thereafter, the above-mentioned light brine was supplied to a separation tower (15) in which 2.4 m ³ of an amphoteric ion exchange resin (“Diaion DSR01” manufactured by Mitsubishi Chemical Corporation) was packed in a column having an inner diameter of 1200 mm × length of 4000 mm. The sulfate radical was removed by chromatographic grafting. That is, 3.6m ³ of light brine was divided into five times and processed every 0.72m ³ , and the entire amount of 3.6m ³ was processed in one hour. (1) First, 0.72 m ³ of light brine is passed at a flow rate of SV 4.0 h ⁻¹ . (2) Next, the flow of light brine is stopped, and 1.2 m ³ of water is passed at a flow rate of SV 4.0 h ⁻¹ . Thereafter, the operations (1) and (2) are repeated five times. However, the pH of the light brine was adjusted to about 11 before passing.

  The electrical conductivity of the effluent from the separation tower (15) is measured, and a liquid having an electric conductivity of 5.0 S / m or more is recovered and circulated to the dissolution tank (1). Discarded. By this operation, a sulfate radical removal capability of 260 kg / d was obtained, and the sulfate radical concentration of the entire electrolytic plant could be managed.

  The above operation is performed continuously, the supply of the light brine to the chlorine adsorption tower (23) is stopped every three months, the light brine in the chlorine adsorption tower (23) is pushed out and recovered with pressurized air, and then the linear velocity Washing was performed by passing water through the chlorine adsorption tower (23) at 20 m / h for 30 minutes in an upflow. In addition, the quantity of water is 5.3 times by volume ratio with respect to the activated carbon in a chlorine adsorption tower (23). Thereafter, the supply of light brine was resumed. These operations were performed by operating the valves (14a), (28a), (28b), and (28c).

  As a result of continuously performing the above operation, the operation pressure of the separation tower (15) was not increased at the time when two years had passed, and the operation was smooth.

Comparative Example 1:
In Example 1, the operation was performed in the same manner as in Example 1 except that the water washing of the chlorine adsorption tower (23) was omitted. The operating pressure of the separation tower (15) gradually increased, and in the first year, the pressure was about twice that of the initial stage, making it difficult to secure the above-mentioned flow rate SV4.0h ^-1 for light brine. Then, as a result of stopping and checking the operation, the inside of the conduit (25) was clogged with suspended solids. As a result of examining the suspended solids, many halophilic bacteria were observed. In addition, when the resin of the separation tower (15) was extracted and examined, there was no sign of deterioration due to free chlorine.

Process explanatory drawing which shows an example of the electrolysis method of this invention

Explanation of symbols

1: Saturation brine preparation process dissolution tank 6: Purification equipment 7: Electrolysis process electrolysis tank 13: Chlorine deaeration process chlorine degassing tower 15: Degassing process separation tower 23: Chlorine adsorption removal process chlorine adsorption tower

Claims (2)

  Saturated brine preparation process in which alkali metal chloride containing sulfate as an impurity is dissolved in water, ion exchange membrane type electrolysis process to electrolyze saturated brine, and the concentration of alkali metal chloride is reduced from the electrolysis process Of alkali metal chloride including a chlorine deaeration process for degassing chlorine from the fresh brine, and a chlorine adsorption removal process for adsorbing and removing residual chlorine in the light brine derived from the chlorine deaeration process with activated carbon The method for electrolyzing an alkali metal chloride according to claim 1, wherein a water supply means is provided in the chlorine adsorption removal step to reduce the salt concentration of water that contacts the activated carbon in the chlorine adsorption removal step.   After the chlorine adsorption removal process, as a degassing process, light brine and water are alternately passed through a separation tower packed with an ion exchanger, and mainly contain an effluent containing an alkali metal chloride and a sulfate. The electrolytic method according to claim 1, wherein the effluent is separated and recovered, and the former is circulated to the saturated brine preparation step, and the latter is removed outside the circulation system.